Process for changing the valence of a metal of variable valence in an organic solution

ABSTRACT

A process for changing the valence of a metal of variable valence in an organic solution whereby a dispersion is formed by agitating the organic solution with an immiscible aqueous solution and an electric current is passed through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state is desired) or in the anode zone of the cell (if a higher valence state is desired), the cathode and anode zones of the cell being separated by a porous membrane, so that the valence state of the metal becomes higher or lower. When the distribution coefficient of the metal varies with a change in valence, the process also provides a means of effecting a transfer of the metal from the organic solution to the aqueous solution.

States Patent Alfred Schneider Morristown;

Arnold Leslie Ayers, Convent Station, both of NJ.

Apr. 14, 1969 Oct. 26 197 1 Allied Chemical Corporation New York, NY.

inventors Appl. No. Filed Patented Assignee PROCESS FOR CHANGING THE VALENCE OF A METAL OF VARIABLE VALENCIE IN AN ORGANIC SOLUTION 14 Claims, 1 Drawing Fig.

lint. Cl 301k 3/00 lField 01'' Search 204/1.5, 86, 91,130,13l,136;23/339, 340, 341, 343, 344,

Primary Examiner-Reuben Epstein Attorneys-Ernest A. Polin and Birgit E. Morris ABSTRACT: A process for changing the valence ofa metal of variable valence in an organic solution whereby a dispersion is formed by agitating the organic solution with an immiscible aqueous solution and an electric current is passed through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state is desired) or in the anode zone of the cell (if a higher valence state is desired), the cathode and anode zones of the cell being separated by a porous membrane, so that the valence state of the metal becomes higher or lower. When the distribution coefficient of the metal varies with a change in valence, the process also provides a means of effecting a transfer of the metal from the organic solution to the aqueous solution.

PROCESS FOR CHANGING Tll-IE VALENQE OF A METAL OlF VARIABLE vAlLlENClE IN AN ORGANIC SOLUTION This invention relates to a process for changing the valence of a metal in organic solution. More particularly, the invention relates to a process for changing the valence of a metal in organic solution by electrochemical means.

BACKGROUND OF THE INVENTION Various commercial processes require a metal of variable valence in organic solution to be in a particular valence state. For example, tetravalent uranium can be precipitated from organic solution with HF. Since the uranium is generally in a higher valence state, it must be reduced to its tetravalent state prior to precipitation.

In another process, mixtures of trivalent plutonium and hexavalent uranium in organic solution can be separated from each other by selective extraction with an immiscible aqueous solution whereby the uranium remains in the organic phase and the plutonium transfers to the aqueous phase. Since the plutonium is generally in its tetravalent state, it must be reduced to the trivalent state prior to separation.

It is known that such changes in valence can be effected by adding another metal such as iron or aluminum, as a reducing agent. While effective to selectively reduce the metal, contamination of the solution with the added metal results. The contaminant metal must then be removed, thereby adding to the cost of such process.

It is also known that the addition of tetravalent uranium to mixtures of hexavalent uranium and tetravalent plutonium in organic solution reduces the plutonium to its trivalent state. This method eliminates the addition of contaminant metal but has the disadvantage that the added uranium may change the isotopic composition of the hexavalent uranium product. Further, the addition of uranium also adds materially to the cost ofsuch process.

US. Pat. No. 3,361 ,65l discloses that tetravalent plutonium in a dilute nitric acid solution with hexavalent uranium can be reduced to its trivalent state electrolytically. This process has the disadvantage that the metals must be in aqueous solution. If the metals are in organic solution, they must, therefore, be first extracted with the nitric acid solution.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for changing the valence ofa metal of variable valence state in organic solution.

It is another object to provide a process for separating metals of variable valence in organic solution by electrochemical means.

It is a further object to provide an improved method of separating plutonium from uranium in organic solution.

Further objects will become apparent in the following detailed description thereof.

We have found that the valence of one or more metals of variable valence in an organic solution can be changed electrochemically by forming a dispersion by agitating said organic solution and an immiscible aqueous solution and passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state is desired) or in the anode zone (if a higher valence state is desired), the anode and cathode zones being separated by a porous membrane, to effect a change in valence state of one or more of the metals.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic sectional view of an electrolytical cell separated into two zones by a porous membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Broadly, the present process is applicable to change the valence of a single metal in an organic solution so that it can be further processed. It is also applicable to change the valence of a metal in organic solution and effect a transfer of the metal to an immiscible aqueous solution in a single step. When the distribution coefficient of the metal is affected by a change in valence and the aqueous solution is a preferential solvent for the metal in its reduced or oxidized state, the immiscible solutions are separated and the aqueous solution will be enriched in the metal in its new valence state whereas the organic solution will be depleted in that metal. In still another application of the invention, organic solutions containing two or more metals of variable valence, one of which has a distribution coefficient affected by a change in the valence, can be treated electrochemically and the metals separated from each other in a single step. After electrochemical treatment, one of the metals will have transferred to the aqueous solution and the other metal(s) will remain in the organic solution.

Since organic solutions are generally nonconducting, it was highly surprising and unexpected to discover that the valence of metals in organic solutions could be changed by electrochemical means.

The process of the invention can be used to reduce hflib avalent uranium to tetravalent uranium in an organic solution by forming a dispersion of the organic solution by agitating with a dilute nitric acid solution in the cathode zone of an electrolytic cell separated into an anode zone and a cathode zone by a porous membrane and passing a current through the cell. Tetravalent uranium in the organic solution will be obtained.

The process can also be used to reduce tetravlaent plutonium to trivalent plutonium in an organic solution and extract the trivalent plutonium from the organic solution by forming a dispersion by agitating the organic solution and a dilute nitric acid solution in the cathode zone of an electrolytic cell and passing a current through the cell. The tetravalent plutonium will be reduced to trivalent plutonium which is preferentially soluble in and hence extracted by the dilute nitric acid. After electrochemical treatment, the organic solution will be depleted in plutonium whereas the aqueous solution will be enriched in plutonium in its trivalent state.

The process is also suitable for the selective reduction of a tetravalent plutonium to trivalent plutonium in an organic solution containing tetravalent plutonium and hexavalent uranium obtained during the processing of nuclear fuels. lPlutonium can be separated from the uranium in such an organic solution by forming a dispersion by agitating the organic solution and a dilute nitric acid solution in the cathode zone of an electrolytic cell and passing a current through the cell. The tetravalent plutonium is reduced to trivalent plutonium, which is preferentially soluble in and transfers to the dilute nitric acid solution. When the organic and aqueous phases are separated, most of the plutonium will be found in the aqueous phase and most of the uranium will remain in the organic phase. Thus, the plutonium can be separated form the uranium rapidly, in a single step without the addition of any reducing agents.

The process can also be employed in purifying metal solutions. Organic solutions containing uranium, plutonium, or both, and metallic impurities in trace quantities, such as ruthenium, zirconium, niobium and the like, can be treated by forming a dispersion with a dilute nitric acid solution in the anode zone of the cell. When the organic and aqueous phases are separated after electrochemical treatment, the trace impurities will be found in the aqueous phase.

The present process can of course also be applied to oxidation reactions by forming a dispersion by agitating an organic solution with an immiscible aqueous solution in the anode compartment of the electrolytic cell and passing a current through the cell. For example, an organic solution containing tetravalent plutonium and tetravalent neptunium can be treated to provide an organic solution containing most of the plutonium and an aqueous solution containing most of the neptunium in its pentavalent state.

Solutions containing from about 5 to percent by weight of an alkyl phosphate, optionally in an organic diluent. are

generally employed as the organic solvent for the metals. The alkyl phosphates can be mono-, di-, or triesters of phosphoric acid derived from alkanols containing one to about eight carbon atoms such as butanol, hexanol, octanol and the like. Tributyl phosphate in an amount of from about 20 to 40 percent by weight in a hydrocarbon diluent is generally employed as solvent for metals found in nuclear fuels, i.e., uranium, plutonium and neptunium, due to its high extraction selectivity. The diluents can be hydrocarbons such as dodecane, kerosene, gasoline and the like. Other organic solvents including ketones, such as hexone or amines such as dioctylamine, the latter in a suitable diluent, can also be employed.

The aqueous solution must be immiscible with the organic solution and must be an electrolyte. Suitable aqueous solutions are dilute mineral acid or salt solutions, such as nitric acid, sodium nitrate and the like. The aqueous solution can also contain a stabilizer, such as hydrazine, to prevent reoxidation of the metals. The hydrazine can be present in the aqueous solution in an amount of from 0.01 to about 0.5 M.

Referring to the FIGURE, the electrolytic cell, employed in carrying out the present invention is conventional and its exact size, shape, and the like can be varied and does not form part of the invention. The cell as further described below is arranged for a reduction operation. The cell 1 is divided into an anode zone 2 and acathode zone 3 by a porous membrane 4. The membrane 4 can be an inorganic porous membrane such as alumina, or an organic ion exchange membrane. [on exchange membranes are generally available as an anionic or cationic exchange resin in a film forming matrix such as polyethylene or a vinyl resin. The anode zone 2 contains an anode 5 and the cathode zone 3 contains a cathode 6. The electrodes can be of conventional materials such as platinum, tantalum, niobium, carbon and the like. Platinum is preferred for the electrodes. The cell 1 is also fitted with inlet ports, 7, 8 and 9 and exit ports 10 and 111. The cathode zone 3 is also fitted with a stirrer 12 which is adequate to effect and maintain a dispersion ofa mixture of organic and aqueous solutions in the cathode zone 3. The cell 1 can also be fitted with means of cooling and/or heating, externally or internally (not shown).

The anode zone 2 contains an aqueous solution of an electrolyte and the cathode zone is partially filled with the same or a different electrolyte. The ratio by volume of organic to electrolyte solutions through which a current can be effectively passed varies somewhat depending on the degree of dispersion of the immiscible phases. The electrolyte must be maintained as the continuous phase during the process. It will be understood that when a reduction is to be carried out, the roles of the anode and the cathode zones are reversed.

In atypical procedure, and organic solution of a metal of variable valence is charged to the anode (if a higher valence state of the metal is desired) or the cathode zone if a lower valence state of the metal is desired) of an electrolytic cell partially filled with an immiscible aqueous solution. Agitation is started to effect a dispersion of the immiscible phases at such a rate so as to maintain the electrolyte as the continuous phase, and the power is then turned on. When the reaction is complete, the dispersion is drawn off and the immiscible solutions are allowed to separate. The aqueous solution can be recycled to the cell and contacted with a fresh batch of the organic solution, either in a batch or semicontinuous manner, to increase the concentration of the metal in the aqueous phase. This method is discussed in greater detail in applicants copending application Ser. No. 815,713 Electrochemical Concentration of Metallic Solutions, filed concurrently herewith.

When the concentration of the metal in the organic solution to be oxidized or reduced is quite low, the rate ofthe oxidation or reduction decreases and longer reaction times are required to complete the reaction. In order to increase the rate of reaction an internal reduction oxidation (redox) agent can be added to the system. Such redox agent is reduced or oxidized by the passage of a current through the cell. When the metal whose valence state is to be changed is to be reduced, the redox agent is reduced, in turn reduces the metal whose valence is to be reduced, and is itself oxidized back to its original valence state. The concentration of this agent will remain substantially constant. The addition of an internal redox agent is particularly effective when added to dilute solutions of tetravalent plutonium. A small amount of uranyl nitrate [UO (NO is generally added to the organic solution as the redox agent. During the reaction, the hexavalent uranium is reduced to tetravalent uranium, which reduces the tetravalent plutonium, and is then reoxidized to hexavalent uranium according to the equation:

2 PU+ U*"- 2 PU* U* The rate of this reaction is rapid and increases the rate of reduction of plutonium. The choice of uranium is particularly convenient in this instance since the hexavalent uranium and any tetravalent uranium will remain in the organic phase while most of the trivalent plutonium will transfer to the aqueous phase. Thus, the addition of the redox agent in this case will not contaminate the trivalent plutonium solution. This process is particularly useful for effecting the transfer of plutonium from an organic to an aqueous solution as is required for the purification processes employing solvent extraction, e.g. the Purex type process.

This invention will be further illustrated by the examples given below. The reactions in the examples is followed by spectrophotometric and radiochemical analyses to determine the valence state of the metals. Standard curves were established for hexavalent uranium at 410 millimicrons, for tetravalent uranium at 645 millimicrons, for trivalent plutonium at 560 and 605 millimicrons, for tetravalent plutonium at 476 millimicrons, for hexavalent plutonium at 831 millimicrons, for tetravalent neptunium at 715 millimicrons and for pentavalent neptunium at 617 millimicrons.

EXAMPLE 1 A measured volume of 0.36 M. uranyl nitrate in an organic solution of 30 volume percent of tributyl phosphate in kerosene also containing 0.3 M nitric acid is charged to the cathode zone of an electrolytic cell containing an equal volume of an aqueous solution of 2 M nitric acid and 0.5 M of hydrazine. The anode zone is filled with an aqueous solution of 2 M nitric acid. The anode zone is fitted with a platinum electrode, sealed into one end. The anode zone and the cathode zone are divided by a permeable cation exchange membrane. The cathode zone is fitted with a platinum electrode and is also provided with a stirrer and an internal cooling coil. The stirrer is turned on to form a dispersion in the cathode zone, and a current density of 0.1 a./sq.cm. at a potential of 7.3 volts is applied to the cell. The hexavalent uranium is reduced to tetravalent uranium. After 2 hours 65 percent of the uranium is reduced. By increasing the acidity of the aqueous phase, the proportion of tetravalent uranium in the organic phase increases.

EXAMPLE 2 EXAMPLE 3 An organic solution containing 82 grams per liter of uranyl nitrate and 1 gram per liter of tetravalent plutonium nitrate in 30 volume percent of tributyl phosphate and kerosene is charged to the cathode zone of the electrolytic cell of example 1, containing an equal volume of an aqueous solution of 2.5 M nitric acid and 0.1 M of hydrazine. A dispersion is formed, and

EXAMPLE 4 An organic solution of 30 volume percent of tributyl phosphate in kerosene containing 6 mg./ml. of tetravalent neptunium is charged to the anode zone of the electrolytic cell of example 1 containing an equal volume of an aqueous solution of l M nitric acid. A dispersion is formed and current density of 0.1 a./sq.cm. at a potential of 11 volts is applied. After 3 hours nearly all of the neptunium is present in the pentavalent state in the aqueous phase.

EXAMPLE 5 An organic solution containing 6 mg./ml. of tetravalent plutonium and 6 mgJml. of tetravalent neptunium in 30 volume percent of tributyl phosphate in kerosene is charged to the anode zone of the electrolytic cell of example 1 containing an equal volume of an aqueous solution of 2 M nitric acid. The current density applied is 0.1 a./sq.cm. at a potential of 11 volts. After 3 hours almost all of the neptunium is oxidized to the pentavalent state and transfers to the aqueous phase, whereas the concentration of plutonium in the organic phase remains substantially the same.

We claim:

l. A process for changing the valence of a metal of variable valence state in an organic solution which comprises forming a dispersion by agitating the organic solution with an immiscible aqueous solution, as electrolyte, and, while maintaining the electrolyte as the continuous phase, passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state ofthe metal is desired) or in the anode zone of the cell (if a higher valence state of the metal is desired), the cathode and anode zones of the cell being separated by a porous membrane, whereby the valence state of the metal becomes lower or higher.

2. A process according to claim ll wherein the organic solution contains uranium and from about 5 to 100 percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent.

3. A process according to claim 2 wherein the aqueous solution is a dilute nitric acid solution.

4. A process according to claim 3 wherein the aqueous solution additionally contains hydrazine.

5. A process for extracting a metal of variable valence state from an organic solution containing the metal which comprises forming a dispersion by agitating the organic solution with, as electrolyte, and immiscible aqueous solution which is a preferential solvent for the metal in a lower or higher valence state and, while maintaining the electrolyte as the continuous phase, passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state of the metal is desired or in the anode zone of the cell (if a higher valence state of the metal is desired), the cathode and anode zones of the cell being separated by a porous membrane, whereby the valence state of the metal becomes lower or higher and the metal transfers from the organic solution to the aqueous solution, and seperating the organic solution from the immiscible aqueous solution.

6. A process according to claim 5 wherein the organic solution contains plutonium and from about 5 to percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent and the aqueous solution is a dilute nitric acid solution.

7. A process according to claim 6 wherein the dispersion is present in the cathode zone and hexavalent uranium is added as a reduction-oxidation a ent.

8. A process for separating metals of variable valence states in an organic solution which comprises forming a dispersion by agitating the organic solution with, as electrolyte, an immiscible aqueous solution which is a preferential solvent for one of the metals in a lower or higher valence state and, while maintaining the electrolyte as the continuous phase, passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state of said metal is desired) or in the anode zone of the cell (if a higher valence state of said metal is desired), the cathode and anode zones of the cell being separated by a porous membrane, whereby the valence state of said metal becomes higher or lower and said metal transfers from the organic solution to the aqueous solution, and separating the organic solution from the immiscible aqueous solution.

9. A process according to claim 8 wherein the organic solution contains uranium and plutonium, from about 5 to 100 percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent and the aqueous solution is a dilute nitric acid solution.

it). A process according to claim 9 wherein the aqueous solution additionally contains hydrazine.

lll A process according to claim 8 wherein the organic solution contains hexavalent uranium and tetravalent plutonium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution, and the dispersion is charged to the cathode zone of the cell, whereby the uranium remains in the organic solution and the plutonium is reduced to trivalent plutonium which transfers :to the aqueous solution.

12. A process according to claim 8 wherein the organic solution contains tetravalent plutonium and tetravalent neptunium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the plutonium remains in the organic solution and the neptunium is oxidized to pentavalent neptunium which transfers to the aqueous solution.

113. A process according to claim 8 wherein the organic solution contains hexavalent uranium and tetravlent neptunium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the uranium remains in the organic solution and the neptunium is oxidized to pentavalent neptuniun which transfers to the aqueous solution.

M. A process according to claim 8 wherein the organic solution contains hexavalent uranium or tetravlent plutonium or both and a metallic impurity in trace quantities selected from the group consisting of ruthenium, zirconium, niobium and mixtures thereof, the aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the trace metals transfer to the aqueous solution. 

2. A process according to claim 1 wherein the organic solution contains uranium and from about 5 to 100 percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent.
 3. A process according to claim 2 wherein the aqueous solution is a dilute nitric acid solution.
 4. A process according to claim 3 wherein the aqueous solution additionally contains hydrazine.
 5. A process for extracting a metal of variable valence state from an organic solution containing the metal which comprises forming a dispersion by agitating the organic solution with, as electrolyte, and immiscible aqueous solution which is a preferential solvent for the metal in a lower or higher valence state and, while maintaining the electrolyte as the continuous phase, passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state of the metal is desired) or in the anode zone of the cell (if a higher valence state of the metal is desired), the cathode and anode zones of the cell being separated by a porous membrane, whereby the valence state of the metal becomes lower or higher and the metal transfers from the organic solution to the aqueous solution, and separating the organic solution from the immiscible aqueous solution.
 6. A process according to claim 5 wherein the organic solution contains plutonium and from about 5 to 100 percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent and the aqueous solution is a dilute nitric acid solution.
 7. A process according to claim 6 wherein the dispersion is present in the cathode zone and hexavalent uranium is added as a reduction-oxidation agent.
 8. A process for separating metals of variable valence states in an organic solution which comprises forming a dispersion by agitating the organic solution with, as electrolyte, an immiscible aqueous solution which is a preferential solvent for one of the metals in a lower or higher valence state and, while maintaining the electrolyte as the continuous phase, passing an electric current through the dispersion in the cathode zone of an electrolytic cell (if a lower valence state of said metal is desired) or in the anode zone of the cell (if a higher valence state of said metal is desired), the cathode and anode zones of the cell being separated by a porous membrane, whereby the valence state of said metal becomes higher or lower and said metal transfers from the organic solution to the aqueous solution, and separating the organic solution from the immiscible aqueous solution.
 9. A process according to claim 8 wherein the organic solution contains uranium and plutonium, from about 5 to 100 percent by weight of an alkyl phosphate optionally containing a hydrocarbon diluent and the aqueous solution is a dilute nitric acid solution.
 10. A process according to claim 9 wherein the aqueous solution additionally contains hydrazine.
 11. A process according to claim 8 wherein the organic solution contains hexavalent uranium and tetravalent plutonium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution, and the dispersion is charged to the cathode zone of the cell, whereby the uranium remains in the organic solution and the plutonium is reduced to trivalent plutonium which transfers to the aqueous solution.
 12. A process according to claim 8 wherein the organic solution contains tetravalent plutonium and tetravalent neptunium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the plutOnium remains in the organic solution and the neptunium is oxidized to pentavalent neptunium which transfers to the aqueous solution.
 13. A process according to claim 8 wherein the organic solution contains hexavalent uranium and tetravalent neptunium and from about 20 to about 40 percent by weight of an alkyl phosphate in a hydrocarbon diluent, the immiscible aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the uranium remains in the organic solution and the neptunium is oxidized to pentavalent neptuniun which transfers to the aqueous solution.
 14. A process according to claim 8 wherein the organic solution contains hexavalent uranium or tetravalent plutonium or both and a metallic impurity in trace quantities selected from the group consisting of ruthenium, zirconium, niobium and mixtures thereof, the aqueous solution is a dilute nitric acid solution and the dispersion is charged to the anode zone of the cell, whereby the trace metals transfer to the aqueous solution. 